1. Field of Invention
This invention relates to the formation of buried layers in semiconductor devices by means of ion implantation, and more specifically to the formation of arsenic implanted subcollectors in silicon devices.
The subcollector is a highly doped buried layer which has the effect of shunting the high resistivity collector region in a bipolar transistor, thereby reducing the series resistance significantly.
Several methods have been used so far for fabricating these subcollectors. The most known is the so-called "closed capsule diffusion". According to that technique the impurities such as Arsenic are diffused into an exposed region of a P type silicon substrate through a masking oxide layer to form a heavily doped N.sup.+ type low resistivity region known as being the "subcollector" of a bipolar transistor. Arsenic is widely used for that purpose because this region must remain localized during subsequent heat treatment steps. The impurity used must meet the two following requirements: have a low diffusion coefficient compared to that of other impurities such as boron and phosphorous currently used in the semiconductor device processing; and have a high solubility limit in order to dope silicon in excess of 10.sup.19 at/cm.sup.3 and form the subcollector with the desired low resistivity.
After the step of forming the subcollector, the oxide masking layer is removed, and the N type collector is grown epitaxially on the substrate. The N.sup.+ subcollector is therefore sandwiched between the N type epitaxial layer and the P type substrate. Formation of base and emitter regions follows in a known manner.
Although the closed capsule process is well established today and is even considered as being pure routine, the development of new bipolar technologies where both emitter and base regions are formed according to ion implantation techniques, has created the need for fully implanted devices to receive benefit from the great advantages offered by ion implantation in terms of: dose control, profile flexibility, process automation, safety, etc. . . . as compared to diffusion.
One may expect significant advantages in replacing the closed capsule diffusion by ion implantation:
closed capsule process exhibits known disadvantages such as: capsule implosion due to evacuation and quenching, As powder contamination, backside coating of wafer, etc. . . .
closed capsule process is a higher temperature and longer process as compared to ion implantation. In addition, the high dose of arsenic near, to the solubility limit (10.sup.21 at/cm.sup.3) which is necessary to form the subcollector in the closed capsule process, is replaced by a low dose (about 2.times.10.sup.16 at/cm.sup.2) in the ion implantation process; the latter also presents the advantage of a higher degree of control.
However, the technology conversion from capsule to ion implantation is not without difficulties. Problems stem from compatibility issues with the capsule process relative to process operation and product performance. Competing with a process (closed capsule) that has matured to a high level of manufacturing stability presents a significant challenge to solve many difficulties.
A first difficulty to be surmounted comes from the fact that implanting impurities such as As at the high doses that are required for forming subcollectors, normally results in damages in the semiconductor crystal lattice. These damages are so severe that amorphous regions are produced which are not completely healed after the subsequent standard annealing step, leaving unacceptable residual crystallographic disorder.
Another difficulty comes from the current practice of implanting through a relatively thin passivating material, such as silicon dioxide, having a thickness of about 250 .ANG..
The advantages provided by the use of this thin, so called "screen layer", are well known; particularly, it acts as a cap; i.e. none of the silicon substrate is exposed at any point to contaminants in the ambient. However, contaminating ions are trapped in the screen layer. These contaminating ions, in a large part, result from collisions between the dopant ions (e.g. As) of the beam and the different parts (sidewalls, apertures, . . .) of the ion implantation equipment. During subsequent high temperature thermal steps such as diffusion and oxidation, those contaminating ions which have been trapped in the screen layer tend to diffuse into the substrate and then contaminate the implanted region.
2. Description of the Prior Art
There exists a very abundant literature relating to this subject, for example:
relating specifically to the problem of the formation of damaged regions.
U.S. Pat. No. 3,895,965, assigned to Bell Telephone Labs, discloses a method of forming buried layers by ion implantation, which includes removal of the damaged region of the semiconductor crystal resulting from such implants. Impurity ions are implanted near the surface of a silicon substrate. The substrate is then heated in an oxidizing ambient for a sufficient length of time to allow the impurities to diffuse further into the crystal while an oxide layer grows on the surface, consuming the damaged region. The oxide is removed leaving the impurities in defect-free material upon which may be grown an epitaxial layer. According to the teaching of this reference, the problem of contamination is not mentioned.
relating specifically to the problem of contamination
U.S. Pat. No. 3,945,856, assigned to International Business Machines, Inc., which discloses a method of ion implantation into a semiconductor substrate which comprises forming a layer of an electrically insulative material, such as silicon dioxide, on the substrate over the region to be ion implanted. Then, a beam of ions having sufficient energy to pass through the layer of insulative material and to penetrate into the substrate, is directed at a particular portion of the insulative layer. Before proceeding further, at least the upper half of the insulative layer, and preferably all of the upper portion of the insulative layer, in excess of a remaining thickness of 100 .ANG., is removed by etching. Then, the substrate is heated whereby the ions are driven further into the substrate to form the selected ion implanted region. This reference does not address at all, the problem of defects usually created in the epitaxial layer.
The above patent provides a general description of the process steps required for fabricating ion implanted arsenic subcollectors, however it does not provide the necessary details for this fabrication to occur on a level approaching "zero defects".
relating to both problems
IBM TDB Vol. 19 No. 3 August 1976, pp 865-865, an article entitled: "Forming Buried Subcollector by Ion Implantation", and,
IBM TDB Vol. 19 No. 8 January 1977, pp 3051-3052, an article entitled: "Elimination of Stacking Faults" by K. D. Beyer et al; and,
IBM TDB Vol. 20 No. 3 August 1977, pp 1003-1004, an article entitled: "Lower Defect Densities in Implanted As Subcollector Devices" by K. D. Beyer et al.
All these references address the problem of the formation of ion implanted subcollectors through a thin screen layer, while minimizing the number of defects such as stacking faults, pipes, etc. . . . which are inherent to the ion implantation process.
Maintaining a competitive edge in semiconductors requires the full solution of these problems before reaching all of the technical and economical advantages of ion implantation which have not yet been fully tapped: improved process yields, higher productivity and, most importantly, significant increases in end of line product yield which in turn result in a very favorable situation from a cost point of view.
It is, therefore, a first object of the present invention to replace the closed capsule subcollector process by an ion implanted process with a significant product yield improvement due to the full suppression while, still maintaining device performance.
It is another object of the present invention to provide a method for forming a buried subcollector in a semiconductor substrate by ion implantation without any contamination and damages in the implanted region.
A final and specific object of the invention is to provide an improvement in the method of forming a subcollector in a silicon semiconductor device by reconstituting the oxide screen layer which was partially etched away to remove the arsenic contaiminants caused by the ion implantation to thus provide a thickened oxide barrier to prevent out-diffusion of the arsenic during the subsequent annealing step.